The hybrid vehicle generally has two different power sources simultaneously. According to the various connection types, it may be mainly divided into three structures of series hybrid, parallel hybrid and series-parallel hybrid.
The series hybrid is characterized in electrically coupled, wherein the engine directly drives an electric generator for charging the battery, and then an electric motor is operated by electrical energy from the battery to drive the wheels. The parallel hybrid is characterized in mechanically coupled, wherein the engine is connected with the driving shaft, and the motor can be selectively operated as an electric motor or a electric generator for balancing the loads of the engine.
In the prior art, one of the series-parallel hybrid may use a planetary gear unit to realize the distribution of power among the driving units. In this structure, the engine may be connected to the carrier of the planetary gear unit, a part of the power of the engine is transferred into the electrical energy by means of the first motor for furnishing the electrical energy to the second motor or for charging the battery, and the other part of the power of the engine is directly transmitted to the ring gear of the planetary gear unit; meanwhile, the second motor is connected with the ring gear for the supply of some supplementary power or torque. In this structure, a part of the power output from the engine is always used to power the first motor to generate the electrical energy for charging the battery or for driving the second motor.
In the prior art, a series-parallel hybrid may use the clutch to realize the power distribution among the driving units. For example, the Chinese patent CN 1200824 C discloses a hybrid power drive system for propelling the vehicle, which comprises two clutches, two motors and a battery, wherein the first clutch and the second clutch are respectively disposed between the first motor, the second motor and the internal combustion engine, the second motor is permanently connected to a variable-ratio transmission, and the first motor and the second motor are connected electrically with the battery. In this patent, because the second motor is permanently connected to the variable-ratio transmission, that is, the second motor is permanently connected with the wheels via the transmission, it has the disadvantage in that: when the second motor fails to work due to some reasons (e.g. the electrical power is low, the second motor is breakdown or is controlled to stop) while the engine is started to drive the wheels, the engine will also drive the rotor of the second motor to rotate because the second motor cannot be disconnected with the wheel driving shaft. Thus, the second motor becomes to be a load to the driving system and, as a result, the energy is wasted and the driving efficiency is lowered.
Moreover, as it can be seen from the disclosure, the first motor of the drive system has three functions as below: (1) starting motor for the internal combustion engine; (2) generator for outputting electrical energy; and (3) drive device for driving the transmission hydraulic pump and other auxiliaries such as an air conditioner compressor, a power steering pump. Thus it can be seen that, the first motor is equal to a starting motor (with lower power or capacity) for the internal combustion engine in a conventional vehicle, except that it is electrically connected with the battery, and under control, it has some other functions besides to start the internal combustion engine, for example, to drive some auxiliaries, but it cannot be operated to drive the wheels alone or together with the second motor. Therefore, the drive system has limited driving modes and functions, and the power efficiency cannot be optimized. In addition, although the second motor can be operated to drive the wheels in a purely electrical manner, it can not work always in a high efficiency when it is used to drive the wheels in various conditions. If it intends to improve the efficiency, the demand for the design of the motor is high and difficult to meet.